The correct option is C Codon
Exon is a part of gene that will become a part of final mature RNA produced by that gene after introns have been removed by RNA splicing.
Cistron is the smallest unit of genetic material that must be intact to function as a transmitter of genetic information.
A codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid (or stop signal) during protein synthesis.
Thus, the correct answer is option (C).